There are several ways to detect of a person a person sitting in a car seat with a sensor system arranged in a car seat. The sensor may be a passive sensor system (capacitors etc) or a dynamic sensor system (piezoelectric sensors etc).
WO0064709 relates to a single capacitive sensor element which is excited by a broadband drive signal and generates a measurement signal which is then subjected to an autocorrelation with the drive signal. The drive signal is preferably a binary pseudo-random noise signal, so that a good signal/noise ratio can be obtained. With a binary drive signal and measurement signal the autocorrelatlon can be carried out, in the simplest of cases, with a simple AND element.
U.S. Pat. No. 5,232,243, discloses an occupant sensing apparatus includes a piezoelectric film sensor and a controller. The piezoelectric film sensor is disposed in a seat and is oscillated by a driving circuit. The controller is connected to the piezoelectric film sensor and monitors an oscillation value of the piezoelectric film sensor. When a person is seated on the seat, the piezoelectric film sensor contacts a contacting arm of the seat. Therefore, the oscillation value of the piezoelectric film sensor is attenuated. As a result, the controller determines a seated condition of the seat in accordance with the oscillation value of the piezoelectric film sensor.
U.S. Pat. No. 5,896,090 discloses an occupant sensing apparatus for detecting a seated person on a seat includes a pressure switch disposed in the seat. The pressure switch has a pair of films and a pair of electrodes disposed between the films. The pair of electrodes are spaced in the films so as to define an OFF condition of the pressure switch. The pressure switch is in an ON condition when the electrodes contact each other. The occupant sensing apparatus further includes a controller connected to the electrodes of the pressure switch and determining whether the seat is occupied in accordance with the ON or OFF condition of the pressure switch. The pressure switch includes a pair of films and a plurality of electrode pairs. The films are made of resin material. Both films are formed in a lattice shape and are thin so as to be flexible. The film is laminated onto the film and the films are connected with a plurality of bond members. The electrodes are disposed between the films and spaced apart from each other. The plurality of electrodes are printed onto an inner surface of the film which is bonded to an inner surface of the film. The plurality of electrodes are printed onto the inner surface of the film which is bonded to the inner surface of the film so as to be able to contact the opposing plurality of electrodes. The plurality of bond members are disposed between electrodes and adjacent electrodes. The bond members are bonded to the inner surface of the film and the inner surface of the film in order to define a distance A between the plurality of electrodes and the plurality of electrodes. When the distance is maintained, the pressure switch is in an OFF condition. When at least one of the electrodes contacts the opposing electrode, the pressure switch is in an ON condition.
Initial tests for in-car presence (occupancy) detection with a single sensor and simple (unfiltered) signal strength threshold comparator method based on prior art systems and methods have often failed, as expected. This is mainly due to the fact that the in-car environment has background vibrations (hereafter referred also as noise) in so great magnitude variations that a more sophisticated algorithm must be designed for reliable presence (occupancy) detection.